1. Field of the Disclosure
The present disclosure relates generally to apparatus and methods for determining a refractive index of a fluid and one or more properties or characteristics of the fluid therefrom, such as fluid recovered from subsurface formations.
2. Description of the Related Art
During both drilling of a wellbore and after drilling, fluid (oil, gas and water) from the formation is often extracted to determine the nature of the hydrocarbons in hydrocarbon-bearing formations. Fluid samples are often collected in sample chambers and the collected samples are tested to determine various properties of the extracted formation fluid. To drill a well, drilling fluid is circulated under pressure greater than the pressure of the formation in which the well is drilled. The drilling fluid invades or penetrates into the formation surrounding the wellbore to varying depths, referred to as the invaded zone. The drilling fluid contaminates the original (virgin) fluid present in the invaded zone. To collect samples of the original fluid present in the formation, a formation testing tool is often conveyed into the wellbore. A pump typically extracts the fluid from the formation via a sealed probe placed against the inside wall of the wellbore. A fluid identification device is typically utilized to determine the contamination level in the fluid. When the fluid is initially extracted, it contains high amounts of the drilling fluid filtrate. The extracted fluid is typically discarded into the wellbore until the fluid identification device measurements indicate that an acceptably low level of contamination has been achieved. Refractometers have been utilized to determine or infer the contamination level in the formation fluids during extraction. Current downhole refractometers are typically based on measurements related to the reflection of light at a window-fluid interface, such as the critical angle of reflection or the intensity of the reflection (at near-normal incidence). Such refractometers are primarily sensitive to the interface region between the fluid and a transparent window (often sapphire) immersed in the fluid, which interface region is only few microns (wavelengths of light) of the fluid beyond the refractometer window that is immersed in the fluid of interest. Often, the refractometer window accumulates a thin film of deposits from the formation fluid. The refractive index measurements then become less reliable because they, at least in part, depend upon the refractive index of this film, and, if the film is thick enough, they only depend on the refractive index of the film.
The disclosure herein provides a refractometer that is substantially unaffected by certain deposits on the window and may be more reliably utilized in downhole tools for determining characteristics of a formation fluid.